The ring signal is generated inside the modem, so a master socket isn't required. I haven't tried it, but I doubt if there's a ring signal on 2 and 5 out of the modem.
If I'm wrong, and there still is a AC ring signal superimposed on 2 and 5, then the cap. in the master would still couple it to 3.
B.T wouldn't have connected the house extension sockets unless they were previously fitted by B.T.
Yes, I fitted a socket next to where the hub is and I made up a short double ended B.T plug to B.T plug lead. (431a to 431a)
You can buy them, search BT male to BT male.
They used to fit NTE5 same as BT, they usually where fitted side by side. I just wired a link across both front plates, to divert the extension wiring between the two.
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